Automatic gain control may be used to balance sensitivity/accuracy in certain electronic components, e.g., analog-to-digital converters, with high signal level tolerance. Consider for example the simplified block diagram of a receiver shown in FIG. 1 which includes automatic gain control functionality. The total gain in the receiver is set to achieve high sensitivity but at the same time handle the necessary, high signal level, blocking requirements so that saturation or maximum signal levels of electronic components in the receiver are not exceeded. For example, the nominal blocking level for the input to the analog-to-digital converter (ADC) 12 is close to the ADC's full scale (FS) value. If an input signal exceeds the nominal blocking level of the ADC 12, the ADC 12 “clips” that signal causing high distortion levels harmonically related to the frequency of that input signal. While clipping is problematic in all receivers, it is particularly problematic in wideband multi-carrier radio receivers where many frequencies are simultaneously received. Those frequency channels received at a low level may be blocked or otherwise seriously distorted by the harmonics from the clipped, high level signal. Ultimately, this effect degrades the performance of the receiver. If such a receiver is used for example in a radio base station in a cellular communications system, that decrease in receiver performance ultimately decreases the cell capacity.
To prevent this kind of undesirable behavior, the receiver in FIG. 1 maybe equipped with an automatic gain controller 14 that detects when the received signal level Pin, detected for example by a reverse-biased diode coupler 16 and converted to digital format (this analog to digital conversion is not shown), exceeds a predefined threshold related to the full scale level of the analog-to-digital converter 12. In this case of analog detection 16 performed prior to analog filtering (not shown) and attenuation 12, there is a sufficient time delay introduced by the filtering which allows the gain of the amplifier 10 to be adjusted before the high level signal Pin, reaches the variable amplifier/attenuator 10 and the analog-to-digital converter 12. By reducing/attenuating the high level signal, clipping is avoided even if the power level increases rapidly.
Gain adjustment of the received signal may also be performed digitally in the digital gain adjustment block 18 which processes the digital input signal received from the analog-to-digital converter 12 and the ADC controller 14 in accordance with the AGC control signal received from AGC controller 14. In cellular communications, the time derivative of the power level of a transmitted signal, e.g., power ramp-up profile, is typically limited. Typically, the time constant involved in the digital detection process performed in the AGC controller 14 is at least an order of magnitude smaller than the time constant governing the power ramp-up of the signal. As a result, digital detection by the AGC controller 14 and digital gain adjustment 18 suffices for cellular radio receiver applications.
Despite the benefits of the AGC controller's ability to reduce (or increase) the gain of the analog variable amplifier 10, each change of gain causes a transient. Transients input to narrowband channel filters cause ringing, and thereby, increase distortion levels and the bit error rate.
To make the transceiver transparent to the change in analog gain, the reduction in analog gain (or the increase in analog gain) may be compensated in the digital domain by the digital gain adjustment block 18 with a corresponding increase (decrease) in digital gain. The ideal AGC response for an input signal with an increasing power level is shown in FIG. 2. The input signal Pin, is shown as a solid, dark black line with a steady ramp-up followed by a leveling. The output of the adjustable amplifier 10 (corresponding to the input of the analog-to-digital converter 12) PADC increases up to a threshold with an abrupt decrease in gain corresponding to the “gain step,” followed by a continued increase with a second gain reduction and so forth. After the digital compensation, however, the output level Pout from the digital gain adjustment block 18 (shown as a dashed line) corresponds to the input signal shape but at a higher signal level.
Although threshold hysteresis may be implemented in the AGC controller to prevent rapid transitions between gain levels by keeping the gain level constant for a minimum dwell time at each gain state, transients still occur even with digital compensation. For example, there may be a mismatch in time between the change of analog gain and digital gain due to inaccurate time calibration. In addition, the time constant of the variable gain analog amplifier 10 and any analog filter(s) determines the analog gain as a function of time, i.e., the slope of the gain change as seen by the digital gain adjustment block 18. As a result, the digital gain adjustment block 18 must be able to compensate for the exact gain profile at all points in time, which requires complicated calibration and a sophisticated digital gain adjustment block 18. A transient may also be caused if there is a gain mismatch between the variable gain analog and digital amplifiers.
Certain measures may be taken to eliminate or reduce such transients. First, mismatch of analog and digital gain may be reduced using digital correction values determined through off-line calibration and stored in a lookup table. A disadvantage with this approach is that such mismatch compensation occur at the sampling rate of the analog-to-digital converter. Second, a transient due to inexact matching of analog and digital gain as a function of time may be reduced by controlling the rate of gain increase (or decrease), i.e., gain ramping. The slope of the gain ramp is controlled to be sufficiently low so that the bandwidth of the variable gain analog amplifier and any filter(s) do not affect the transient behavior. As a result, the variable gain analog and digital functions can be matched in time, and any residual mismatch in time causes only a low level transient since the slope of the gain function is relatively small. A disadvantage is that this approach requires a sophisticated digital gain block working at the full rate of the analog-to-digital converter. Third, a mismatch in time may be minimized by delay calibration of the AGC control loop. The time between ordering the analog gain change until its effect is seen at the analog-to-digital converter output is measured. The measured delay is then used to calculate when to initiate digital gain compensation after the analog gain has been changed.
For ease of implementation and to achieve low complexity operations at the full sampling rate of the analog-to-digital converter, the digital gain compensation in block 18 is preferably achieved by simple bit shifts. However, this limits the gain to be a step function with a minimum amplitude change of approximately 6 dB (one bit). An increase of one bit is equivalent to a doubling of the possible amplitude; hence, the change in dB is 20 log 2=6.02 dB. As a result, a transient is likely to be caused every time there is a gain adjustment, even with the transient compensation approaches described above. Thus, to completely avoid AGC transients, complex calibration and computationally costly operations at the highest sampling rate in the receiver are required. Any less computationally intensive AGC scheme results in transients which increase the bit error rate of the receiver, and therefore, have a negative impact on the receiver's performance.
The present invention overcomes the problems identified above by employing diversity receiving techniques, which often already exist in many current receivers, particularly in cellular radio communication systems. Diversity processing is used in the present invention to permit low complexity, automatic gain controllers to be employed in the diversity receiver branches which require only a minimum of calibration but do not sacrifice system performance.
The present invention reduces automatic gain control (AGC) transients using first and second AGC processing branches to receive a signal. If the AGC thresholds in the first and second AGC branches are exceeded, and assuming for example that the AGC threshold in the first AGC branch is first exceeded, the gain in the first AGC processing branch is selected for adjustment during a first time period. However, the gain in the second AGC processing branch is not adjusted during that first time period. The signals generated by the first and second AGC processing branches are then diversity processed to generate a received signal. The diversity processing effectively selects the branch currently without gain adjustment and thereby reduces the effect of any AGC transient. One way of diversity processing is to base branch selection or weighting on signal-to-noise-and-distortion for each branch.
If the threshold of the second AGC branch is first exceeded for a particular time period, its gain would be adjusted, and the gain of the first AGC branch would not be adjusted during that time period. Other criteria could be used to determine which of the branches should be adjusted (or which should not be adjusted) for a particular time period assuming the AGC thresholds for those branches are exceeded. Moreover, the first and second thresholds may be the same or different.
In any event, the signal output from the selected, AGC processing branch where the gain was not adjusted during the time period reduces the effect of an AGC adjustment transient caused by any adjustment of the gain applied to the signal in the first AGC processing branch during that time period.
Apparatus in accordance with the present invention includes two (or more) receiving branches each receiving a signal. Each receiving branch includes an amplifier and automatic gain controlling circuitry for selectively adjusting the gain applied to the signal by the amplifier. Control circuitry selects an automatic gain controller in one of the receiving branches to adjust (if desired) the gain of the corresponding amplifier for a period of time. In particular, the gain of only one of the amplifiers may be adjusted during that time period. That time period is preferably set in accordance with an automatic gain control transient settling time. The receiving branches each include an analog-to-digital converter, and may also include digital gain adjustment circuitry.
Advantageously, the automatic gain controllers in the receiving branches require only minimum amounts of calibration and may be low complexity. In a preferred example embodiment, each branch may use an analog N×6 dB, (where N is an integer), step amplifier/attenuator, and the digital gain compensation may be implemented by simple bit shifting.
Accordingly, the present invention is well-suited for use in most radio communications systems, and particularly, in wideband, multi-carrier, cellular radio receivers, such as those found in cellular radio base stations.